(1) Field of the Invention
This invention relates to cable grips and concerns grips of the kind which, in use, take the form of a woven, open-mesh flexible tube which is used securely to enclose a cable or cables along at least part of the length thereof.
(2) Description of the Prior Art
Such cable grips can be used to pull a cable into a required position or to support a cable in a desired position or both. To facilitate their use they are usually provided at their draft or leading end with one or a pair of loops or eyes. For example, if a cable is to be fixed in position with its longitudinal axis having a substantial vertical component, the cable grip would be arranged on the cable with the eyes (taking the case where a pair of eyes is provided) at or near the end of the cable which is to be uppermost. A lifting member can be arranged to engage the eyes and pull the grip and cable upwardly until the cable is in the required position and then the eyes can be secured to a support. Thus the weight of the cable is taken by the grip and support, reducing or preventing longitudinal strain in the conductors in the case of an electric cable, and absorbing vibration and flexural forces. The cable grip can be used indoors or outdoors on any suitable structure such as a building or pole, down an excavation, in a mine shaft and elsewhere.
Cable grips are manufactured in two basic forms; the `closed` form and the `split` form.
The closed cable grip is made in tubular form from two sets of helically disposed and interwoven wire strands of opposite hand. To prepare the grip for use it is axially compressed, and thus radially expanded, and pushed in this condition tail first over the end of the cable until it is in its required position. The axial compression is then relieved, causing the grip to contract radially and, as the normal internal diameter of the grip is less than the diameter of the cable, grip it lightly. If then the grip is pulled by the eyes in the opposite direction to that in which it was pushed on, the tension in the individual strands forming the open-mesh lattice tries to cause a further reduction in its diameter so that it grips the cable tightly and then pulls the cable in the said opposite direction. This kind of grip is used when a free end of the cable is available for introduction into the tail end of the cable grip. Examples of such a grip are those sold under Nos. 022-01-A and 022-06 in the Catalogue of Harvey Hubbell Ltd. Cable grips are usually of the so-called endless weave type and woven from lengths of wire which are bent at their mid-points to form loops. As weaving is started at the tail end remote from the eyes the grip terminates at this end in a series of loops which limits the amount by which the diameter of the tail end can be increased by endwise compression of the grip. Thus one disadvantage of the closed cable grip of a given nominal diameter is that the open tail end remote from the eyes can be increased by only a relatively small amount although the diameter of the tube at points intermediate its ends can be increased by progressively greater amounts up to a point substantially equidistant from the ends. Accordingly if there is a discontinuity in the length of cable, such as a connector having a relatively large diameter with respect to that of the cable, then the open tail end cannot be enlarged sufficiently to pass over it.
It has been assumed in the above description that the grip is of the `constant weave` type, that is the mesh of the lattice is of constant size and shape throughout the length of the grip. However there is another type of grip, known as the `variable weave` type in which, at the tail end, the mesh size is smaller or the angles at the longitudinal corners of the diamonds formed by the intersecting strands of the mesh are greater, than at the leading or draft end. The advantage thought to be gained by the adoption of a variable weave grip is that, when a force is applied to the eyes, the tail end of the grip is first to undergo a radial contraction which advances progressively towards the leading end of the grip, thus ensuring maximum frictional engagement with the cable throughout the length of the grip. However the disadvantage of variable weave grips is that, because of the limitations imposed by the higher weave angles and/or smaller mesh at the tail end, the increase in its diameter that can be gained by axial compression is even more restricted than in the constant weave type and thus the problem of enlarging the tail end to pass over a discontinuity is exacerbated. In such cases, and in the case where a free end of cable is not available, it is necessary to use a `split` form of cable grip.
The split cable grip is similar in principle to the closed grip but is formed as an elongate mesh which is wrapped around the cable, or cables, and then the longitudinal edges are joined together as by lacing with a strand or strands, usually, of a similar material or by means of a rod which is passed through preformed loops on the longitudinal edges of the mesh. Examples of such grips are those sold under Nos. 022-02-A and 022-03-A in the catalogue of Harvey Hubbell Ltd. The split cable grip can accommodate discontinuities of larger diameter than the cable but has the disadvantage that it has to be `laced-up` in position and this can be time-consuming and laborious especially in confined spaces: for example, one such use of the cable grip is for preventing strain in the cables and their connectors to mining equipment in underground pit shafts and tunnels. A further disadvantage of the split cable grip is that it is less strong than a comparable closed cable grip.
There has thus been a long-standing requirement for a cable grip which has the advantages of the closed grip but which can accommodate relatively large discontinuities along the length of the cable.